January 7, 2015

In Tiny Ear Bones, the Life Story of a Giant Bluefin Tuna

By Anne Cohen, Graham Lane :: Originally published online August 27, 2004

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The Atlantic bluefin tuna, Thunnus thynnus, is one of the fastest,
most powerful and most beautiful of fish. It is also the most expensive. Highly
prized by sushi connoisseurs, a single giant fish of 1,400 pounds may sell
for $40,000.

The tuna’s high price has
led the fishery to the brink of collapse. In 1981,
in response to declining
numbers of tuna, the International Commission for Conservation
of Atlantic Tunas (ICCAT) introduced a strict management
policy for Atlantic bluefin that rapidly developed
into one of the most controversial and politically
charged issues in fisheries management.

The policy controversy, familiar
to both commercial and recreational New England fishermen,
centers on
the assumption that there are two discrete and independent
North Atlantic populations. The two populations are
arbitrarily divided into eastern and western territories
at the 45°W meridian. Each presumed stock is subject
to different management restrictions, the most striking
of which is the imposition of a strict, near-zero harvest
quota for the western stock and the absence of country-specific
quotas for the eastern stock.

There is considerable debate concerning the appropriateness
of the two-stock division because evidence is lacking
to support its two key assumptions. The first is that
eastern and western tuna populations reproduce separately
in separate spawning grounds, with western fish spawning
in the Gulf of Mexico and eastern fish in the Mediterranean.
The second is that the tuna populations do not migrate
across the Atlantic and intermingle.

A large research effort is currently
underway to test these assumptions by tracking the
movements of individual
fish across the North Atlantic and studying their spawning
behavior. Much of this effortled by Barbara
Block of Stanford University and Molly Lutcavage of
the New England Aquariumhas involved the use
of sophisticated pop-up satellite tags.

Pop-up satellite tags currently
have limited life-spans, ranging perhaps from months
to
years.
At Woods Hole
Oceanographic Institution, we are investigating the
feasibility of using chemical signatures in the otoliths,
or ear bones, of giant fish to obtain information about
trans-Atlantic migrations, stock mixing, and spawning
habitats. The entire and detailed life history, from
birth to death, of a giant 30-year-old bluefin is contained
within a single otolith, or ear bone, less than one
inch long.

Our approach is based on the premise that differences
in water chemistry and temperature experienced by fish
during their oceanic travels will be recorded as distinct
and predictable changes in the trace elements of aragonite,
the mineral that makes up the otolith. This approach
differs from most previous otolith studies in our use
of microbeam technology to track chemical changes at
weekly to daily resolution, within a single ear bone.

Using the micron-scale sampling capabilities of the
Cameca 3f ion microprobe and techniques developed to
study coral skeletons, we have been able to analyze
the chemical composition of the primordium, a region
of otolith just 20 microns in diameter. The primordium
forms when the fish is still in the larval stage, and
its chemical composition contains a record of where
the fish was born.

Our initial results are promising and show that we
may be able to use chemical signatures in the primordium
to distinguish different populations of bluefin
tunain their first years of life when the primordium is being formed.
With conventional bulk sample analyses, we are not able to distinguish between
different stages (i.e. larval, juvenile, adult) of otolith formation. By contrast,
our approach gives us the ability to reconstruct the life history of the fish
from birth to death.

Because we can obtain a daily,
rather than an average, record of the tuna’s
travels, we may be able to tell when, during the
tuna’s long life, it
swam in which waters. We can potentially discern whether
the tuna was born in the west and migrated east, or
was born in the east and migrated west, instead of
knowing only that it was in both areas sometime during
its life. This will improve our ability to manage populations
of this magnificent fish.

Images

Chemical clues within the 2-centimeter-long ear bones of bluefin tuna are giving scientists the ability to track which waters the fish swam in and reconstruct their life histories. (Illustration by E. Paul Oberlander, WHOI)

Woods Hole Oceanographic Institution is the world's leading non-profit oceanographic research organization.
Our mission is to explore and understand the ocean and to educate scientists, students, decision-makers, and the public.